Ion-amplified photochemical filtering drives plastic carbon transformation into redox-active dissolved organic matter

Microplastics represent a rapidly expanding global anthropogenic carbon reservoir, yet the mechanisms governing the transformation of plastic carbon into environmentally reactive dissolved organic matter (DOM) under ionic environments remain poorly understood. Here, this study investigate the photochemical transformation of DOM from non-biodegradable polyethylene (PE) and biodegradable poly(butylene adipate-co-terephthalate) (PBAT) microplastics in ultrapure and ionic water matrices. Ultrahigh-resolution molecular evidence reveals a novel “ion-amplified photochemical filtering” effect framework, that profoundly reorganizes plastic-derived DOM molecular pools. Ionic exposure sharply reduces molecular formula richness from 320 and 422 to 145 and 262 for PE and PBAT, respectively. The ion-filtered DOM pools retain polymer-specific molecular signatures, with PE-derived DOM dominated by high H/C aliphatic moieties, while PBAT-derived DOM exhibits higher oxygenation levels. This ionic filtering drives consistent molecular focusing, concentrating DOM toward lipid-, protein-, and amino sugar-like domains with a constrained carbon oxidation state near − 1.25. Photochemical aging simplifies DOM structures, lowers aromatic density, and enhances ROS generation. PBAT-derived DOM exhibits higher redox reactivity than PE-derived DOM, further verified by Cr(VI) probing. This work establishes a formula-level ionic filtering framework that reshapes plastic carbon into redox-active DOM, advancing mechanistic understanding of microplastic biogeochemical cycling in natural aquatic environments.